It is known that endotoxin which is an outer membrane component of various Gram negative bacilli has various biological activities, such as enhancement of immune function of an animal. The main active portion of the endotoxin is considered to reside in a disaccharide moiety which is one of units constituting lipopolysaccharide, and establishment or presumption of structures of disaccharide moiety of various Gram negative bacilli has been studied. For example, the structure of the disaccharide moiety of Salmonella typhimurium was established as reported in J. Biol. Chem., 258, 12801 to 12803 (1983), and the presumed structure of the disaccharide moiety of Escherichia coli was reported in Tetrahedron Lett., 24, 4017-4020 (1983).
In parallel to these studies, various disaccharide derivatives having structures similar to the established or presumed structures of the natural disaccharide compounds (hereinafter referred to as natural lipid A) have been synthesized, and their biological activities, such as immunological activity, have been examined. However, any of these synthetic disaccharide derivatives (hereinafter referred to as synthetic lipid A) lacks biological activities or has, if any, far less activity as compared with natural lipid A extracted from E. coli (Infec. Immun., 45, 293-296 (1984) and Eur. J. Biochem., 140, 221-227 (1984).)
The above described differences between the natural lipid A and the synthetic lipid A on biological activities are believed to arise from differences of fatty acid residues bonded to the 2'-amino group and 3'-hydroxyl group of a non-reducing end of glucosamine. More specifically, in the known synthetic lipid A, a straight chain fatty acid residue or a straight chain fatty acid residue having a hydroxyl group at the 3-position is bonded to the 2'-amino group and the 3'-hydroxyl group of the non-reducing end of glucosamine. Whereas, in the natural lipid A, it is presumed that a straight chain fatty acid residue having a hydroxyl group at the 3-position is bonded to the 2'-amino group and the 3'-hydroxyl group, and the fatty acid moiety has a straight chain fatty acid residue bonded to the 3-hydroxyl group thereof through an ester linkage. Lipid A extracted from E. coli has not yet been isolated and identified as a single component. (Nihon Saikingaku Zasshi, 39 (3), 295 and 463 (1984))